Since 1988, the National Institute of Child Health and Human Development (NICHD) has provided funding for a support services contract to conduct the Mothers and Infants Cohort Study (MIS) and the Women and Infants Transmission Study (WITS). These studies have focused on developing an understanding of the mechanisms by which women infected with HIV transmit their infection to their infants and the effects of HIV exposure on those infants, both among the infected and uninfected children. The studies have also examined the progression of disease in the women over time. The ascertainment of the immunologic, neurologic and neurodevelopmental problems and serious bacterial and opportunistic infections resulting from HIV infect ion in these studies is critical to developing appropriate measures of disease progression in clinical trials of new anti-retroviral drugs in HIV-infected children. The Mothers and Infants Cohort Study (MIS) has completed enrollment of its cohort of pregnant women and is currently focusing on major data analysis efforts under this new contract. While the initial clinical findings are expected to be completed in the next year, the follow-up of the children and the analysis of the wealth of data collected during the past five years of the project is expected to take approximately three to four more years to complete. The Women and Infants Transmission Study (WITS) continues to enroll HIV-infected pregnant women and has shifted its primary focus from determining the epidemiologic risk factors for transmission of infection from mother to infant to the determination of the timing of transmission (intrauterine versus intrapartum) and to methods of early diagnosis of infection in the infant. The ability to accurately determine the infection status of exposed infants at an earlier time will allow for pursuit of early intervention strategies to prevent progression of disease in infected infants, while avoiding potential exposure of uninfected infants to toxic therapies. The subject contract will continue to provide the support and analytical services required for the two studies.